FIG. 1 is a schematic diagram of the existing GSM network architecture, as shown in FIG. 1, the traditional Global System for Mobile Communication (GSM) network architecture comprises a core network, a Base Station Controller (BSC) and a Base Station (BS), wherein the base station controller and the base station also can be collectively called as the Base Station Subsystem (BSS). The interface between the core network and the BSC is called as the A interface, the interface between the BSC and the BTS is called as the Abis interface, and the interface between the BTS and the Mobile Station (MS) is called as the Um interface; the core network, BSC and the BTS cooperatively work, and provides the complete mobile voice service function to the MS.
With the evolution of the technique, the flat GSM network architecture appears, FIG. 2 is a schematic diagram of the flat GSM network architecture, and as shown in FIG. 2, the flat GSM network mainly comprises two layers, i.e. the core network and the Enhanced Base Station (eBTS), wherein the interface between the core network and the enhanced base station is the A interface, the interface between the enhanced base station and the mobile station is the Um interface, and a new interface between the enhanced base stations is defined, which is called as the interface between enhanced base stations and can be used for the interworking between the enhanced base stations. Although the traditional GSM system network architecture and the flat GSM network architecture are the same in the actions of the air interface, one of advantages of the flat GSM network architecture is that it can use the same architecture with the similar flat enhanced High Speed Packet Access+(HSPA+) or the Long Term Evolution (LTE) system network to operate in the co-spectrum, and this co-spectrum operation of the GSM with the HSPA+ or the LTE will cause that the operator wishes to dynamically share the limited spectrum resources in the different wireless techniques (GSM, HSPA+, and LTE and so on), wherein one application scenario possibly appearing is that: the HSPA+ or LTE occupies a majority of spectrum resources, whereas the GSM occupies a minority of spectrum resources, and the amount of the GSM users will not dramatically decreases due to the cause of the communication habit. Thus, the GSM network is required to provide the voice service for these users in relative less spectrum resources, and therefore, a huge challenge is brought to the traditional GSM technique. Since the traditional GSM is the Time Division Multiple Access (TDMA) system, the system capacity is limited to the Time Slot (TS) resources, whereas one TS is only able to bear one Full Rate (FR) voice user or two Half Rate (HR) voice users at the same time.
With the continuous evolution of the communication technique, many techniques for improving the spectrum utilization ratio appear, one of which is the technique of the Voice services over adaptive Multi-user channels on One Slot (VAMOS), and the technique of the voice services over adaptive multi-user channels on one slot permits to bear two FR voice users or 4 HR voice users on one slot at the same time, thereby improving the support of the whole GSM network for the voice user capacity without increasing the spectrum resources.
However, if the technique of the voice services over adaptive multi-user channels on one slot is applied, it certainly will increase the mutual interference among users in the same one slot and cause the reduction of the signal receiving quality. Since the control channel, for example the Slow Associated Control Channel (SACCH), bears very important control information (the information such as power adjustment, frame adjustment and measurement data and so on), a problem in the SACCH transmission will be caused once the interference increases, a great influence on the communication between the MS and the BTS will be caused, and a severe one will be a call drop.
FIG. 3 is a schematic diagram of four mobile stations multiplexing the HR channel resources in the GSM mode, as shown in FIG. 3, in the traditional GSM mode, two HR users—user 1 and user 2 multiplex the HR carrier 1, and other two HR users—user 3 and user 4 multiplex the HR carrier 2. When the HR carrier 1 and the HR carrier 2 are different frequencies, the SACCH channels among users will not interfere with each other. In the mode of the voice services over adaptive multi-user channels on one slot, four HR users are multiplexed to one slot, then the HR carrier 1 and the HR carrier 2 are the identical frequency, they use the identical SACCH frame mapping way (the traditional mapping way), and SACCH channels of the user 1 and the user 3 and the SACCH channels of the user 2 and user 4 will become the identical frequency interference to each other, whereas since the SACCH is always transmitted with the maximal power, the interference problem among the control channels of different mobile station users will become severe.
In order to improve the SACCH channel performance, the Shifted-SACCH technique appears. FIG. 4 is a schematic diagram of the four mobile stations using the shifted-SACCH frame mapping way to multiplex the HR channel resources in the GSM mode, as shown in FIG. 4, HR users—user 3 and user 4 use the technique called as the shifted-SACCH frame mapping, and this technique makes the SACCH frames of users in the subcarrier 1 of the voice services over adaptive multi-user channels on one slot and the users in the subcarrier 2 of the voice services over adaptive multi-user channels on one slot to be mapped into different time slots, namely they do not appear at the same time, and thus the interference is impossibly formed between them.
Besides, when a certain one or more users are in the silence period of the Discontinuous Transmission (DTX), the Traffic Channel (TCH) corresponding to the user will turn off the transmission power, thus the SACCHs of other users which are situated at the same voice services over adaptive multi-user channels on one slot will not be interfered, thereby improving the SACCH channel performance and ensuring the communication quality.
The appearance of the shifted-SACCH frame mapping technique also cause one problem, namely two frame mapping ways will exist in the network using the technique of the voice services over adaptive multi-user channels on one slot, namely the traditional frame mapping way and the shifted-SACCH frame mapping way, and the mobile terminal is required to obtain the frame mapping way used by a call when establishing this call, as it not only could use the traditional frame mapping way, but also could use the shifted-SACCH frame mapping way. Therefore, it is required to provide a method to make the mobile terminal to be able to obtain which frame mapping way is used when establishing a call.